Various tools are used in hydrocarbon exploration and production to measure properties of geologic formations during or shortly after the excavation of a borehole. The properties are measured by formation evaluation (FE) tools and other suitable devices, which are typically integrated into a bottomhole assembly. Sensors are used in the FE tools to monitor various downhole conditions and formation characteristics.
It is important that sensor information data be correlated with associated depths to provide an accurate representation of the formation. True vertical depth (TVD) is the vertical distance between a downhole location and the surface, and is useful in associating with various measurement data.
Numerous techniques for measuring TVD have been utilized. Depth measurements may be estimated by measuring the length of a drillstring of a formation evaluation/exploration system. However, various environmental factors may cause the drillstring to be stretched or shortened, such as load changes, friction phenomena and thermal influences on the drillstring, so that additional measurement data is required to calculate TVD. Significant factors for an elongation of drill pipe are drill pipe stretch and thermal expansion, and thus measurement of these factors is important to increase the accuracy in depth correction and associated TVD calculations.
The measured depth of a downhole tool is normally an estimation based on an assumed stiff and inelastic drill pipe, which is corrected by measurement and drill string model data. Such drillstring models provide information about the elongation of the drill pipe caused by load changes, thermal influences or other environmental factors. Normally parameters to calibrate these models are measured only on the surface and/or at one point downhole (in a BHA) and linear distribution of the environmental factors is assumed along the length of the drillstring and/or TVD. These models do not take into account changes or fluctuations in such factors along the length of the drillstring, and thus do not provide a completely accurate representation of the effects of such factors.